The present invention relates to a developing apparatus for the development of a latent image formed on an image forming body used for an electrophotographic image forming apparatus.
Compared with 1-component-developer composed of magnetic toner without using magnetic carrier, 2-component-developer, in which toner and magnetic carrier are mixed, is advantageous in that: the toner can be easily subjected to triboelectric charging control; and the toner is seldom coagulated. Therefore, when 2-component-developer is used, toner transfer can be effectively controlled by a development electric field. Accordingly, where a color image is developed by color toner, high image quality can be provided. Consequently, 2-component-developer is frequently used although the quantity of toner with respect to carrier must be controlled.
The 2-component-developer is used in the following developing apparatuses: a developing apparatus in which a developing sleeve is fixed, and a magnet, in which a plurality of N and S poles are circumferentially disposed, is rotated in the developing sleeve; a developing apparatus in which a developing sleeve and a magnet are rotated together; and a developing apparatus in which a developing sleeve is rotated, and the poles of a magnet provided in the developing sleeve are fixed. In a developing apparatus in which the magnet provided in the developing sleeve is rotated, high torque must be applied to the magnet to rotate at high speed. Therefore, vibrations are caused, and the rotational mechanism is complicated, and further the apparatus must be strong and large. For this reason, a developing apparatus, in which a developing sleeve is rotated, and the poles of a magnet provided in the developing sleeve are fixed is widely used. In this developing apparatus with a rotated sleeve and fixed magnet poles, developer is deposited onto the surface of the developing sleeve by the action of the magnet provided in the developing sleeve, and when the developing sleeve is rotated, the developer is conveyed to a developing region disposed at the closest position to an image forming body on which a latent image is formed. In this way, the latent image is developed.
When the latent image on the image forming body is developed, two methods are applied. One is a contact developing method by which development is conducted when a developer layer formed on the developing sleeve rubs the surface of the image forming body. The other is a non-contact developing method by which a latent image is developed when toner particles are scattered in a small gap so as to be deposited onto the latent image on the image forming body while a development bias voltage including an AC component is impressed upon the gap. When a color image is formed by superimposing toner images on the image forming body, it is desirable to employ the non-contact developing method, because a previously formed image is not damaged by the superimposition of the next image. According to the non-contact developing method, the developing operation is conducted in the following manner:
A small gap is formed in the developing region between the image forming body and the developing sleeve. This gap is formed as small as possible. A thin developer layer is formed on the developing sleeve, so that the developer layer is maintained in a non-contact condition with respect to the image forming body. While magnetic carrier of the thin developer layer adheres onto the surface of the developing sleeve, toner particles are separated from the magnetic carrier by the action of an oscillating electric field, and scattered in the gap so that the toner particles adhere onto the latent image to be developed.
The method for forming a thin developer layer on the developing sleeve is disclosed in Japanese Patent Publication Open to Public Inspection Nos. 62-191868 and 62-191869 applied by the present applicant. In the publications described above, a fore end of a resilient plate supported by a support member is directed toward the upstream direction with respect to the conveyance of developer provided on the developing sleeve, and the resilient plate is pressed against the developing sleeve so that the thickness of the developer layer conveyed on the developing sleeve can be regulated. In order to improve the above inventions, the present applicant further proposes Japanese Patent Publication Open to Public Inspection No. 1-282578. In this publication, a rod-shaped member, a portion of which is formed cylindrical, is provided, and the cylindrical portion is pressed against the surface of the developing sleeve so as to form a thin developer layer.
In the non-contact developing method in which a thin developer layer is formed on the developing sleeve surface, the following problems are encountered.
(1) When the developing gap in the developing region is small, the developing properties are affected by the fluctuation of the developing sleeve, wherein the fluctuation is caused by the eccentricity of the developing sleeve. Accordingly, step-like unevenness is caused in the formed image.
(2) When a rod-shaped rigid member is pressed against the developing sleeve to form a thin developer layer, toner particles are fused and deposited on the developing sleeve and the rigid member. Therefore, a uniformly thick developer layer can not be formed, and the developing properties are deteriorated.
(3) In order to suppress the layer thickness of developer, the number of poles of the fixed magnet is increased to restrict the height of developer bristles. Therefore, the magnetic flux density is lowered at a position of the magnet to be used for development, and excess carrier is deposited onto the surface of the image forming body.
It is an object of the present invention to provide a developing apparatus for conducting non-contact development in which the above problems are solved.